Flight Controls - Chapter 5
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    Flight Controls - Chapter 5 Flight Controls - Chapter 5 Document Transcript

    • Chapter 5Flight Controls Introduction This chapter focuses on the flight control systems a pilot uses to control the forces of flight, and the aircraft’s direction and attitude. It should be noted that flight control systems and characteristics can vary greatly depending on the type of aircraft flown. The most basic flight control system designs are mechanical and date back to early aircraft. They operate with a collection of mechanical parts such as rods, cables, pulleys, and sometimes chains to transmit the forces of the flight deck controls to the control surfaces. Mechanical flight control systems are still used today in small general and sport category aircraft where the aerodynamic forces are not excessive. [Figure 5-1] 5-1
    • of this project is to develop an adaptive neural network-based Control stick flight control system. Applied directly to flight control system Elevator feedback errors, IFCS provides adjustments to improve aircraft performance in normal flight as well as with system Pulleys failures. With IFCS, a pilot is able to maintain control and safely land an aircraft that has suffered a failure to a control surface or damage to the airframe. It also improves mission capability, increases the reliability and safety of flight, and eases the pilot workload. Today’s aircraft employ a variety of flight control systems. Cable Push rod For example, some aircraft in the sport pilot category rely on weight-shift control to fly while balloons use a standard burnFigure 5-1. Mechanical flight control system. technique. Helicopters utilize a cyclic to tilt the rotor in theAs aviation matured and aircraft designers learned more about desired direction along with a collective to manipulate rotoraerodynamics, the industry produced larger and faster aircraft. pitch and anti-torque pedals to control yaw. [Figure 5-3]Therefore, the aerodynamic forces acting upon the controlsurfaces increased exponentially. To make the control forcerequired by pilots manageable, aircraft engineers designed Yawmore complex systems. At first, hydromechanical designs, ve Cycli Collecti c Cycliconsisting of a mechanical circuit and a hydraulic circuit, c Cyclic stickwere used to reduce the complexity, weight, and limitationsof mechanical flight controls systems. [Figure 5-2] Yaw Cy clic clic Cy Neutral e Control stick (AFT—Nose up) ectiv Coll LEGEND Hydraulic pressure Hydraulic return Pivot point Anti-torque pedals Collective lever Elevator (UP) Control valves Control cables Figure 5-3. Helicopter flight control system. Neutral For additional information on flight control systems, refer to the appropriate handbook for information related to the Power disconnect linkage Neutral Power cylinder flight control systems and characteristics of specific types of aircraft.Figure 5-2. Hydromechanical flight control system. Flight Control SystemsAs aircraft became more sophisticated, the control surfaces Flight Controlswere actuated by electric motors, digital computers, or fiber Aircraft flight control systems consist of primary andoptic cables. Called “fly-by-wire,” this flight control system secondary systems. The ailerons, elevator (or stabilator), andreplaces the physical connection between pilot controls and rudder constitute the primary control system and are required tothe flight control surfaces with an electrical interface. In control an aircraft safely during flight. Wing flaps, leading edgeaddition, in some large and fast aircraft, controls are boosted devices, spoilers, and trim systems constitute the secondaryby hydraulically or electrically actuated systems. In both control system and improve the performance characteristics ofthe fly-by-wire and boosted controls, the feel of the control the airplane or relieve the pilot of excessive control forces.reaction is fed back to the pilot by simulated means. Primary Flight ControlsCurrent research at the National Aeronautics and Space Aircraft control systems are carefully designed to provideAdministration (NASA) Dryden Flight Research Center adequate responsiveness to control inputs while allowing ainvolves Intelligent Flight Control Systems (IFCS). The goal5-2
    • natural feel. At low airspeeds, the controls usually feel soft Aileronsand sluggish, and the aircraft responds slowly to control Ailerons control roll about the longitudinal axis. The aileronsapplications. At higher airspeeds, the controls become are attached to the outboard trailing edge of each wing andincreasingly firm and aircraft response is more rapid. move in the opposite direction from each other. Ailerons are connected by cables, bellcranks, pulleys and/or push-pull tubesMovement of any of the three primary flight control surfaces to a control wheel or control stick.(ailerons, elevator or stabilator, or rudder), changes the airflowand pressure distribution over and around the airfoil. These Moving the control wheel or control stick to the right causeschanges affect the lift and drag produced by the airfoil/control the right aileron to deflect upward and the left aileron to deflectsurface combination, and allow a pilot to control the aircraft downward. The upward deflection of the right aileron decreasesabout its three axes of rotation. the camber resulting in decreased lift on the right wing. The corresponding downward deflection of the left aileron increasesDesign features limit the amount of deflection of flight control the camber resulting in increased lift on the left wing. Thus,surfaces. For example, control-stop mechanisms may be the increased lift on the left wing and the decreased lift on theincorporated into the flight control linkages, or movement right wing causes the airplane to roll to the right.of the control column and/or rudder pedals may be limited.The purpose of these design limits is to prevent the pilot from Adverse Yawinadvertently overcontrolling and overstressing the aircraft Since the downward deflected aileron produces more lift asduring normal maneuvers. evidenced by the wing raising, it also produces more drag. This added drag causes the wing to slow down slightly. This resultsA properly designed airplane is stable and easily controlled in the aircraft yawing toward the wing which had experiencedduring normal maneuvering. Control surface inputs cause an increase in lift (and drag). From the pilot’s perspective, themovement about the three axes of rotation. The types of yaw is opposite the direction of the bank. The adverse yawstability an airplane exhibits also relate to the three axes of is a result of differential drag and the slight difference in therotation. [Figure 5-4] velocity of the left and right wings. [Figure 5-5] El Rudder—Yaw ev at La or Vertical axis — (lo tera Pit Lift (directional ng l a ch sta i x stability) bil tud is Roll ity ina on— Drag ) l Ailer al itudin Long(lateral axis ity) stabil A Lift g Dra dv e r s e yaw Primary Control Airplane Axes of Type of Figure 5-5. Adverse yaw is caused by higher drag on the outside Surface Movement Rotation Stability wing, which is producing is caused by higher drag on the outside Figure 5-2. Adverse yaw more lift. Aileron Roll Longitudinal Lateral wing, which is producing more lift. Elevator/ Adverse yaw becomes more pronounced at low airspeeds. Pitch Lateral Longitudinal Stabilator At these slower airspeeds aerodynamic pressure on control Rudder Yaw Vertical Directional surfaces are low and larger control inputs are required to effectively maneuver the airplane. As a result, the increase in Figure 5-1. Airplane controls, movement, axes of rotation, and aileron deflection causes an increase in adverse yaw. The yawFigure 5-4. Airplane controls, movement, axes of rotation, and type of stability. is especially evident in aircraft with long wing spans.type of stability. 5-3
    • Application of rudder is used to counteract adverse yaw. Theamount of rudder control required is greatest at low airspeeds, Aileron deflected uphigh angles of attack, and with large aileron deflections. Likeall control surfaces at lower airspeeds, the vertical stabilizer/rudder becomes less effective, and magnifies the controlproblems associated with adverse yaw. Differential aileronAll turns are coordinated by use of ailerons, rudder, andelevator. Applying aileron pressure is necessary to placethe aircraft in the desired angle of bank, while simultaneousapplication of rudder pressure is necessary to counteract the Aileron deflected downresultant adverse yaw. Additionally, because more lift isrequired during a turn than when in straight-and-level flight, Figure 5-6. Differential ailerons.the angle of attack (AOA) must be increased by applyingelevator back pressure. The steeper the turn, the more elevator Neutralback pressure is needed.As the desired angle of bank is established, aileron and rudderpressures should be relaxed. This stops the angle of bank fromincreasing, because the aileron and rudder control surfaces arein a neutral and streamlined position. Elevator back pressureshould be held constant to maintain altitude. The roll-out Raisedfrom a turn is similar to the roll-in, except the flight controlsare applied in the opposite direction. Aileron and rudder areapplied in the direction of the roll-out or toward the high wing.As the angle of bank decreases, the elevator back pressureshould be relaxed as necessary to maintain altitude. DragIn an attempt to reduce the effects of adverse yaw,manufacturers have engineered four systems: differential Loweredailerons, frise-type ailerons, coupled ailerons and rudder,and flaperons.Differential AileronsWith differential ailerons, one aileron is raised a greaterdistance than the other aileron is lowered for a givenmovement of the control wheel or control stick. This produces Figure 5-4. Frise-type ailerons. Figure 5-7. Frise-type ailerons.an increase in drag on the descending wing. The greater dragresults from deflecting the up aileron on the descending wing angles of attack. Frise-type ailerons may also be designedto a greater angle than the down aileron on the rising wing. to function differentially. Like the differential aileron, theWhile adverse yaw is reduced, it is not eliminated completely. frise-type aileron does not eliminate adverse yaw entirely.[Figure 5-6] Coordinated rudder application is still needed wherever ailerons are applied.Frise-Type AileronsWith a frise-type aileron, when pressure is applied to the Coupled Ailerons and Ruddercontrol wheel or control stick, the aileron that is being raised Coupled ailerons and rudder are linked controls. This ispivots on an offset hinge. This projects the leading edge of accomplished with rudder-aileron interconnect springs, whichthe aileron into the airflow and creates drag. It helps equalize help correct for aileron drag by automatically deflectingthe drag created by the lowered aileron on the opposite wing the rudder at the same time the ailerons are deflected. Forand reduces adverse yaw. [Figure 5-7] example, when the control wheel or control stick is moved to produce a left roll, the interconnect cable and spring pullsThe frise-type aileron also forms a slot so air flows smoothly forward on the left rudder pedal just enough to prevent theover the lowered aileron, making it more effective at high nose of the aircraft from yawing to the right. The force applied5-4
    • Rudder deflects with ailerons Figure 5-9. Flaperons on a Skystar Kitfox MK 7. The up-elevator position decreases the camber of the elevator and creates a downward aerodynamic force, which is greater than the normal tail-down force that exists in straight-and- level flight. The overall effect causes the tail of the aircraft to move down and the nose to pitch up. The pitching moment occurs about the center of gravity (CG). The strength of the pitching moment is determined by the distance between the CG and the horizontal tail surface, as well as by the aerodynamic effectiveness of the horizontal tail surface. Moving the control column forward has the opposite effect. In this case, elevator camber increases, creating more lift (less tail-down force) on the horizontal stabilizer/elevator. Rudder/Aileron interconnecting springs This moves the tail upward and pitches the nose down. Again, the pitching moment occurs about the CG.Figure 5-8. Coupled ailerons and rudder. As mentioned earlier in the coverage on stability, power, Figure 5-5. Coupled ailerons and rudder.to the rudder by the springs can be overridden if it becomes thrustline, and the position of the horizontal tail surfacesnecessary to slip the aircraft. [Figure 5-8] on the empennage are factors in elevator effectiveness controlling pitch. For example, the horizontal tail surfacesFlaperons may be attached near the lower part of the vertical stabilizer, at the midpoint, or at the high point, as in the T-tail design.Flaperons combine both aspects of flaps and ailerons. Inaddition to controlling the bank angle of an aircraft likeconventional ailerons, flaperons can be lowered togetherto function much the same as a dedicated set of flaps. Thepilot retains separate controls for ailerons and flaps. A mixeris used to combine the separate pilot inputs into this single Control column Aft Tail downset of control surfaces called flaperons. Many designs that Up elevator Nose upincorporate flaperons mount the control surfaces away from CGthe wing to provide undisturbed airflow at high angles ofattack and/or low airspeeds. [Figure 5-9]Elevator DownwardThe elevator controls pitch about the lateral axis. Like the aerodynamic forceailerons on small aircraft, the elevator is connected to thecontrol column in the flight deck by a series of mechanicallinkages. Aft movement of the control column deflectsthe trailing edge of the elevator surface up. This is usually Figure 5-6. elevator is the primary control for changing the Figure 5-10. The elevator is the primary control for changing thereferred to as up “elevator.” [Figure 5-10] pitch attitude of an airplane. pitch attitude of an airplane. 5-5
    • T-TailIn a T-tail configuration, the elevator is above most of theeffects of downwash from the propeller as well as airflowaround the fuselage and/or wings during normal flightconditions. Operation of the elevators in this undisturbed airallows control movements that are consistent throughout mostflight regimes. T-tail designs have become popular on many CGlight and large aircraft, especially those with aft fuselage-mounted engines because the T-tail configuration removesthe tail from the exhaust blast of the engines. Seaplanes andamphibians often have T-tails in order to keep the horizontalsurfaces as far from the water as possible. An additionalbenefit is reduced vibration and noise inside the aircraft.At slow speeds, the elevator on a T-tail aircraft must be moved Figure 5-11. Airplane with a T-tail design atat hign angle of attack Figure 5-7. Airplane with a T-tail design a a high AOA and anthrough a larger number of degrees of travel to raise the nose aft CG. aft CG. and ana given amount than on a conventional-tail aircraft. This is control stops to elevator down springs. An elevator downbecause the conventional-tail aircraft has the downwash from spring assists in lowering the nose of the aircraft to prevent athe propeller pushing down on the tail to assist in raising the stall caused by the aft CG position. The stall occurs becausenose. the properly trimmed airplane is flying with the elevator in a trailing edge down position, forcing the tail up and the noseSince controls on aircraft are rigged so that increasing control down. In this unstable condition, if the aircraft encountersforces are required for increased control travel, the forces turbulence and slows down further, the trim tab no longerrequired to raise the nose of a T-tail aircraft are greater than positions the elevator in the nose-down position. The elevatorthose for a conventional-tail aircraft. Longitudinal stability of then streamlines, and the nose of the aircraft pitches upward,a trimmed aircraft is the same for both types of configuration, possibly resulting in a stall.but the pilot must be aware that the required control forces aregreater at slow speeds during takeoffs, landings, or stalls than The elevator down spring produces a mechanical load on thefor similar size aircraft equipped with conventional tails. elevator, causing it to move toward the nose-down position if not otherwise balanced. The elevator trim tab balances theT-tail airplanes also require additional design considerations elevator down spring to position the elevator in a trimmedto counter the problem of flutter. Since the weight of the position. When the trim tab becomes ineffective, the downhorizontal surfaces is at the top of the vertical stabilizer, spring drives the elevator to a nose-down position. The nosethe moment arm created causes high loads on the vertical of the aircraft lowers, speed builds up, and a stall is prevented.stabilizer which can result in flutter. Engineers must [Figure 5-12]compensate for this by increasing the design stiffness of thevertical stabilizer, usually resulting in a weight penalty over The elevator must also have sufficient authority to hold theconventional tail designs. nose of the aircraft up during the roundout for a landing. In this case, a forward CG may cause a problem. During theWhen flying at a very high AOA with a low airspeed and landing flare, power is usually reduced, which decreases thean aft CG, the T-tail aircraft may be susceptible to a deep airflow over the empennage. This, coupled with the reducedstall. In a deep stall, the airflow over the horizontal tail landing speed, makes the elevator less effective.is blanketed by the disturbed airflow from the wings andfuselage. In these circumstances, elevator or stabilator control As this discussion demonstrates, pilots must understand andcould be diminished, making it difficult to recover from the follow proper loading procedures, particularly with regardstall. It should be noted that an aft CG is often a contributing to the CG position. More information on aircraft loading, asfactor in these incidents, since similar recovery problems well as weight and balance, is included in Chapter 9, Weightare also found with conventional tail aircraft with an aft CG. and Balance.[Figure 5-11]Since flight at a high AOA with a low airspeed and an aft StabilatorCG position can be dangerous, many aircraft have systems As mentioned in Chapter 2, Aircraft Structure, a stabilatorto compensate for this situation. The systems range from is essentially a one-piece horizontal stabilizer that pivots5-6
    • Antiservo tab Down spring Stabilator Pivot points Elevator Balance weight Pivot point Bell crank Figure 5-13. One stabilator is a one-piece horizontal tail up and Figure 5-9. The piece horizontal tail surface that pivots surface that pivots upaand down about a central hinge point. down about central hinge point.Figure 5-12. When the aerodynamic efficiency of the horizontalFigure 5-8. is inadequate due to an aft CG condition,horizontaltail surface When the aerodynamic efficiency of the an elevator design, and the other with a surface of the same approximatetail surface is inadequate due to an aft center of gravity condition,down spring may be used to supply a mechanical load to lower size and airfoil of the aft-mounted wing known as a tandeman elevator down spring may be used to supply a mechanical loadthe nose. wing configuration. Theoretically, the canard is consideredto lower the nose. more efficient because using the horizontal surface to helpfrom a central hinge point. When the control column is lift the weight of the aircraft should result in less drag for apulled back, it raises the stabilator’s trailing edge, pulling given amount of lift.the airplane’s nose up. Pushing the control column forwardlowers the trailing edge of the stabilator and pitches the nose Rudderof the airplane down. The rudder controls movement of the aircraft about its vertical axis. This motion is called yaw. Like the other primary controlBecause stabilators pivot around a central hinge point, they surfaces, the rudder is a movable surface hinged to a fixedare extremely sensitive to control inputs and aerodynamic surface, in this case to the vertical stabilizer, or fin. Movingloads. Antiservo tabs are incorporated on the trailing edge to the left or right rudder pedal controls the rudder.decrease sensitivity. They deflect in the same direction as thestabilator. This results in an increase in the force required to When the rudder is deflected into the airflow, a horizontalmove the stabilator, thus making it less prone to pilot-induced force is exerted in the opposite direction. [Figure 5-15] Byovercontrolling. In addition, a balance weight is usually pushing the left pedal, the rudder moves left. This alters theincorporated in front of the main spar. The balance weight airflow around the vertical stabilizer/rudder, and creates amay project into the empennage or may be incorporated onthe forward portion of the stabilator tips. [Figure 5-13]CanardThe canard design utilizes the concept of two lifting surfaces,the canard functioning as a horizontal stabilizer located infront of the main wings. In effect, the canard is an airfoilsimilar to the horizontal surface on a conventional aft-taildesign. The difference is that the canard actually creates liftand holds the nose up, as opposed to the aft-tail design whichexerts downward force on the tail to prevent the nose fromrotating downward. [Figure 5-14]The canard design dates back to the pioneer days of aviation,most notably used on the Wright Flyer. Recently, the canardconfiguration has regained popularity and is appearing on Figure 5-14. The Piaggio P180 includes a variable-sweep canardnewer aircraft. Canard designs include two types–one with a design, which provides longitudinal stability about the lateralhorizontal surface of about the same size as a normal aft-tail axis. 5-7
    • sideward lift that moves the tail to the right and yaws the nose When both rudder and elevator controls are moved by theof the airplane to the left. Rudder effectiveness increases with pilot, a control mixing mechanism moves each surface thespeed; therefore, large deflections at low speeds and small appropriate amount. The control system for the V-tail isdeflections at high speeds may be required to provide the more complex than that required for a conventional tail. Indesired reaction. In propeller-driven aircraft, any slipstream addition, the V-tail design is more susceptible to Dutch rollflowing over the rudder increases its effectiveness. tendencies than a conventional tail, and total reduction in drag is minimal. Yaw Secondary Flight Controls Secondary flight control systems may consist of wing flaps, leading edge devices, spoilers, and trim systems. Left rudder forward Flaps CG Flaps are the most common high-lift devices used on aircraft. These surfaces, which are attached to the trailing edge of the wing, increase both lift and induced drag for any given AOA. Flaps allow a compromise between high cruising speed and low landing speed, because they may be extended when needed, and retracted into the wing’s structure when not needed. There are four common types of flaps: plain, split, slotted, and Fowler flaps. [Figure 5-17] Left rudder Ae rod The plain flap is the simplest of the four types. It increases ynamic force the airfoil camber, resulting in a significant increase in the coefficient of lift (CL) at a given AOA. At the same time,Figure 5-11. The effect of left rudder pressure.Figure 5-15. The effect of left rudder pressure. it greatly increases drag and moves the center of pressure (CP) aft on the airfoil, resulting in a nose-down pitchingV-Tail moment.The V-tail design utilizes two slanted tail surfaces to performthe same functions as the surfaces of a conventional elevator The split flap is deflected from the lower surface of the airfoiland rudder configuration. The fixed surfaces act as both and produces a slightly greater increase in lift than the plainhorizontal and vertical stabilizers. [Figure 5-16] flap. More drag is created because of the turbulent air pattern produced behind the airfoil. When fully extended, both plain and split flaps produce high drag with little additional lift. The most popular flap on aircraft today is the slotted flap. Variations of this design are used for small aircraft, as well as for large ones. Slotted flaps increase the lift coefficient significantly more than plain or split flaps. On small aircraft, the hinge is located below the lower surface of the flap, and when the flap is lowered, a duct forms between the flap well in the wing and the leading edge of the flap. When the slotted flap is lowered, high energy air from the lower surface is ducted to the flap’s upper surface. The high energy air from the slot accelerates the upper surface boundary layer and delays airflow separation, providing a higher CL. Thus, theFigure 5-16. Beechcraft Bonanza V35. slotted flap produces much greater increases in maximum coefficient of lift (CL-MAX) than the plain or split flap. WhileThe movable surfaces, which are usually called ruddervators, there are many types of slotted flaps, large aircraft oftenare connected through a special linkage that allows the control have double- and even triple-slotted flaps. These allow thewheel to move both surfaces simultaneously. On the other maximum increase in drag without the airflow over the flapshand, displacement of the rudder pedals moves the surfaces separating and destroying the lift they produce.differentially, thereby providing directional control.5-8
    • Basic section Leading Edge Devices High-lift devices also can be applied to the leading edge of the airfoil. The most common types are fixed slots, movable slats, leading edge flaps, and cuffs. [Figure 5-18] Fixed slot Plain flap Movable slot Split flap Slotted flap Leading edge flap Fowler flap Leading edge cuff Slotted Fowler flap Figure 5-18. Leading edge high lift devices. Fixed slots direct airflow to the upper wing surface and delay airflow separation at higher angles of attack. The slot doesFigure 5-17. Five common types of flaps. not increase the wing camber, but allows a higher maximum CL because the stall is delayed until the wing reaches aFowler flaps are a type of slotted flap. This flap design not greater AOA.only changes the camber of the wing, it also increases thewing area. Instead of rotating down on a hinge, it slides Movable slats consist of leading edge segments, which movebackwards on tracks. In the first portion of its extension, it on tracks. At low angles of attack, each slat is held flushincreases the drag very little, but increases the lift a great deal against the wing’s leading edge by the high pressure that formsas it increases both the area and camber. As the extension at the wing’s leading edge. As the AOA increases, the high-continues, the flap deflects downward. During the last portion pressure area moves aft below the lower surface of the wing,of its travel, the flap increases the drag with little additional allowing the slats to move forward. Some slats, however, areincrease in lift. 5-9
    • pilot operated and can be deployed at any AOA. Opening aslat allows the air below the wing to flow over the wing’supper surface, delaying airflow separation.Leading edge flaps, like trailing edge flaps, are used toincrease both CL-MAX and the camber of the wings. This typeof leading edge device is frequently used in conjunction withtrailing edge flaps and can reduce the nose-down pitchingmovement produced by the latter. As is true with trailing edgeflaps, a small increment of leading edge flaps increases lift toa much greater extent than drag. As greater amounts of flapsare extended, drag increases at a greater rate than lift.Leading edge cuffs, like leading edge flaps and trailing edge Figure 5-19. Spoilers reduce lift and increase drag during descentflaps are used to increase both CL-MAX and the camber of and landing.the wings. Unlike leading edge flaps and trailing edge flaps,leading edge cuffs are fixed aerodynamic devices. In most tabs, balance tabs, antiservo tabs, ground adjustable tabs, andcases leading edge cuffs extend the leading edge down and an adjustable stabilizer.forward. This causes the airflow to attach better to the uppersurface of the wing at higher angles of attack, thus lowering Trim Tabsan aircraft’s stall speed. The fixed nature of leading edge The most common installation on small aircraft is a singlecuffs extracts a penalty in maximum cruise airspeed, but trim tab attached to the trailing edge of the elevator. Most trimrecent advances in design and technology have reduced this tabs are manually operated by a small, vertically mountedpenalty. control wheel. However, a trim crank may be found in some aircraft. The flight deck control includes a trim tab positionSpoilers indicator. Placing the trim control in the full nose-downFound on many gliders and some aircraft, high drag devices position moves the trim tab to its full up position. Withcalled spoilers are deployed from the wings to spoil the smooth the trim tab up and into the airstream, the airflow over theairflow, reducing lift and increasing drag. On gliders, spoilers horizontal tail surface tends to force the trailing edge of theare most often used to control rate of descent for accurate elevator down. This causes the tail of the airplane to movelandings. On other aircraft, spoilers are often used for roll up, and the nose to move down. [Figure 5-20]control, an advantage of which is the elimination of adverseyaw. To turn right, for example, the spoiler on the right wing If the trim tab is set to the full nose-up position, the tab movesis raised, destroying some of the lift and creating more drag on to its full down position. In this case, the air flowing underthe right. The right wing drops, and the aircraft banks and yaws the horizontal tail surface hits the tab and forces the trailingto the right. Deploying spoilers on both wings at the same time edge of the elevator up, reducing the elevator’s AOA. Thisallows the aircraft to descend without gaining speed. Spoilers causes the tail of the airplane to move down, and the noseare also deployed to help reduce ground roll after landing. By to move up.destroying lift, they transfer weight to the wheels, improvingbraking effectiveness. [Figure 5-19] In spite of the opposing directional movement of the trim tab and the elevator, control of trim is natural to a pilot. IfTrim Systems the pilot needs to exert constant back pressure on a controlAlthough an aircraft can be operated throughout a wide range column, the need for nose-up trim is indicated. The normalof attitudes, airspeeds, and power settings, it can be designed trim procedure is to continue trimming until the aircraft isto fly hands-off within only a very limited combination of balanced and the nose-heavy condition is no longer apparent.these variables. Trim systems are used to relieve the pilot of Pilots normally establish the desired power, pitch attitude,the need to maintain constant pressure on the flight controls, and configuration first, and then trim the aircraft to relieveand usually consist of flight deck controls and small hinged control pressures that may exist for that flight condition.devices attached to the trailing edge of one or more of the Any time power, pitch attitude, or configuration is changed,primary flight control surfaces. Designed to help minimize expect that retrimming will be necessary to relieve the controla pilot’s workload, trim systems aerodynamically assist pressures for the new flight condition.movement and position of the flight control surface to whichthey are attached. Common types of trim systems include trim5-10
    • Nose-down trim stabilator moves up, the linkage forces the trailing edge of the tab up. When the stabilator moves down, the tab also moves down. Conversely, trim tabs on elevators move opposite of Elevator the control surface. [Figure 5-21] Trim tab Tab up—elevator down Nose-up trim Elevator Stabilator Trim tab Pivot point Antiservo tab Tab down—elevator upFigure 5-20. The movement of the elevator is opposite to thedirection 5-16. The movement of the trim tab. is opposite to the Figure 5-21. An antiservo tab attempts to streamline the control Figure of movement of the elevator elevator Figure 5-17. An antiservo tab attempts to streamline the direction of movement of the elevator trim tab. surface and is used tois usedthe stabilator less sensitive by opposing control surface and make to make the stabilator less sensitiveBalance Tabs theopposing the force exerted by the pilot. by force exerted by the pilot.The control forces may be excessively high in some aircraft,and, in order to decrease them, the manufacturer may use Ground Adjustable Tabsbalance tabs. They look like trim tabs and are hinged in Many small aircraft have a nonmovable metal trim tab on theapproximately the same places as trim tabs. The essential rudder. This tab is bent in one direction or the other while ondifference between the two is that the balancing tab is coupled the ground to apply a trim force to the rudder. The correctto the control surface rod so that when the primary control displacement is determined by trial and error. Usually, smallsurface is moved in any direction, the tab automatically adjustments are necessary until the aircraft no longer skidsmoves in the opposite direction. The airflow striking the tab left or right during normal cruising flight. [Figure 5-22]counterbalances some of the air pressure against the primarycontrol surface, and enables the pilot to move more easilyand hold the control surface in position.If the linkage between the balance tab and the fixed surface isadjustable from the flight deck, the tab acts as a combinationtrim and balance tab that can be adjusted to any desireddeflection.Antiservo TabsAntiservo tabs work in the same manner as balance tabsexcept, instead of moving in the opposite direction, they movein the same direction as the trailing edge of the stabilator.In addition to decreasing the sensitivity of the stabilator, anantiservo tab also functions as a trim device to relieve controlpressure and maintain the stabilator in the desired position. Figure 5-22. A ground adjustable tab is used on the rudder of manyThe fixed end of the linkage is on the opposite side of the small airplanes to correct for a tendency to fly with the fuselagesurface from the horn on the tab; when the trailing edge of the slightly misaligned with the relative wind. 5-11
    • Adjustable StabilizerRather than using a movable tab on the trailing edge of theelevator, some aircraft have an adjustable stabilizer. With thisarrangement, linkages pivot the horizontal stabilizer about itsrear spar. This is accomplished by use of a jackscrew mountedon the leading edge of the stabilator. [Figure 5-23] Adjustable stabilizer Nose down Nose up Jackscrew Pivot Figure 5-24. Basic autopilot system integrated into the flight control system. Trim motor or trim cable The autopilot system also incorporates a disconnect safety feature to disengage the system automatically or manually. Figure 5-19. Some airplanes, including most jet transports,Figure 5-23. Some airplanes, including most jet transports, use an These autopilots work with inertial navigation systems,adjustable stabilizer to provide the required required pitch trim use an adjustable stabilizer to provide the pitch trim forces. global positioning systems (GPS), and flight computers to forces. control the aircraft. In fly-by-wire systems, the autopilot isOn small aircraft, the jackscrew is cable operated with a trim an integrated component.wheel or crank. On larger aircraft, it is motor driven. Thetrimming effect and flight deck indications for an adjustable Additionally, autopilots can be manually overridden. Becausestabilizer are similar to those of a trim tab. autopilot systems differ widely in their operation, refer to the autopilot operating instructions in the Airplane Flight ManualAutopilot (AFM) or the Pilot’s Operating Handbook (POH).Autopilot is an automatic flight control system that keeps an Chapter Summaryaircraft in level flight or on a set course. It can be directed bythe pilot, or it may be coupled to a radio navigation signal. Because flight control systems and aerodynamic characteristicsAutopilot reduces the physical and mental demands on a pilot vary greatly between aircraft, it is essential that a pilotand increases safety. The common features available on an become familiar with the primary and secondary flightautopilot are altitude and heading hold. control systems of the aircraft being flown. The primary source of this information is the AFM or the POH. VariousThe simplest systems use gyroscopic attitude indicators and manufacturer and owner group websites can also be amagnetic compasses to control servos connected to the flight valuable source of additional information.control system. [Figure 5-24] The number and location ofthese servos depends on the complexity of the system. Forexample, a single-axis autopilot controls the aircraft about thelongitudinal axis and a servo actuates the ailerons. A three-axis autopilot controls the aircraft about the longitudinal,lateral, and vertical axes. Three different servos actuateailerons, elevator, and rudder. More advanced systems ofteninclude a vertical speed and/or indicated airspeed hold mode.Advanced autopilot systems are coupled to navigational aidsthrough a flight director.5-12